Question

how human action causing the death of useful bacteria and fungi

Answer 1

process like global warming,floods,droughts,deforestation,soil erosion are due to human processes which ultimatly result in disturbance of habitats of bacteria and fungi
leading to their death

Answer 2

the release of toxic waste into water and other means cases the death or depletion of the useful fungi and bacteria... this toxic waste contains harmful chemicals which in turn erodes away the microorganisms...you may take a example of ur own body .. there are many useful microbes inside ur intestines.. when u eat some junk food which is harmful to our digestive system, it not only interrupts our lifestyle, it also reduce our immune level..which in turn cause of weaken body , death of useful bacteria..etc. Like that the useful bacteria and fungi are dying.